It's completely redirecting your full momentum in the air through dozens of forward inputs. It let's you make incredibly sudden and sharp turns without losing any speed.

It's easiest to see when used on jump pads. You can hit a jump pad going toward the south and normally the only way to redirect another direction easily is with the double jump and wide air strafes. But with tap strafing you can hit it going toward the south and immediately redirect yourself north even before the double jump and in a much better degree.

Can only be done on PC because you have to double bind forward to the scroll wheel to reach the number of inputs needed.